1. The Field of the Invention
The present invention relates generally to x-ray tubes. More specifically, the present invention relates to an improved method for manufacturing and assembling components of x-ray tubes.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis and testing. While used in a number of different applications, the basic operation of x-ray tubes is similar. In general, x-rays, or x-ray radiation, are produced when electrons are accelerated, and then impinged upon a material of a particular composition.
Typically, this process is carried out within a vacuum enclosure formed as a part of the x-ray tube structure. Disposed within the evacuated enclosure is an electron generator, or cathode, and an anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes electrons to be emitted. A high voltage potential is placed between the anode and the cathode, which causes the emitted electrons to accelerate towards a target surface on the anode. Typically, the electrons are xe2x80x9cfocusedxe2x80x9d into an electron beam towards a desired xe2x80x9cfocal spotxe2x80x9d located on the target surface.
During operation of the x-ray tube, the electrons in the beam strike the target surface (or focal track) at a high velocity. The target surface on the target anode is composed of a material having a high atomic number, and a small portion of the kinetic energy of the striking electron stream is thus converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as a patient""s body. As is well known, the x-rays can be used for such applications as therapeutic treatment, x-ray medical diagnostic examination, or material analysis procedures.
In addition to stimulating the production of x-rays, the kinetic energy resulting from the striking electrons also produces a significant amount of heat in the target anode and surrounding region. As a result, the area of the target anode typically experiences extremely high operating temperatures. At least some of the heat generated in the target anode is absorbed by other structures and components of the x-ray device as well.
In addition, a percentage of the electrons that strike the anode target surface rebound from the surface and then impact other xe2x80x9cnon-targetxe2x80x9d surfaces within the x-ray tube enclosure. These are often referred to as xe2x80x9csecondaryxe2x80x9d electrons. These secondary electrons retain a significant amount of kinetic energy after rebounding, and when they impact these other non-target surfaces, a significant amount of heat is generated in these areas as well. The heat produced by secondary electrons, in conjunction with the high temperatures generated by the primary electron beam, often reaches levels high enough to damage portions of the x-ray tube structure. For example, the joints and connection points between x-ray tube structures can be weakened when repeatedly subjected to such thermal stresses. Ultimately, these conditions can shorten the operating life of the tube, and can affect its operating efficiency, and/or render it inoperable.
The various joints and connection points between various tube components can be especially vulnerable to thermal stresses. For example, during assembly, tube parts comprising differing metallic substances, such as the stem and rotor hub portions of a rotating anode x-ray tube, are typically joined together with a braze joint. A brazing compound, such as a palladium cobalt alloy, is frequently used to join the stem, usually made of TZM (an alloy comprising titanium, zirconium, and molybdenum), to the rotor hub which usually comprises core iron or Ni-base superalloy, such as Incoloy 909. This type of brazing is used to interconnect other x-ray tube components as well.
Conventional brazing materials and procedures, as well as other conventional bonding techniques, suffer from several disadvantages, especially when subjected to the high operating temperatures of an x-ray tube. In general, braze materials have relatively low melting points compared to the materials that are being bonded. Thus, the braze is susceptible to damage from high operating temperatures. Also, brazed joints tend to crack when subjected to the mechanical and thermal stresses present during tube operation. Such cracks usually form in the braze layer itself or in the adjacent substrate material, and may be caused by improper brazing technique, or by an intermetallic layer that often forms as a byproduct during brazing. This intermetallic layer forms along the joint seam and may contain a mixture of any of the compounds that comprise the brazing compound, or of the component materials that are being attached, such as the stem or rotor hub. This intermetallic layer can result in a brittle xe2x80x9cweak linkxe2x80x9d in the joint and, when subjected to the harsh operating conditions within the x-ray tubexe2x80x94which may include temperatures in excess of 850xc2x0 C., a near vacuum, and rotational rates up to about 10,000 revolutions per minute (xe2x80x9crpmxe2x80x9d)xe2x80x94tiny fissures, or xe2x80x9cmicrocracksxe2x80x9d may begin to form in the intermetallic layer or adjacent component material. These microcracks expand over time and cause a gradual weakening of the bond between the components. Such weakening induces instability and wobbling into the rotating anode target that can result in the premature mechanical failure of one or more tube components, or can decrease the operating efficiency of the tube.
Another problem exists with conventional bonding techniques, especially when used to join a stem to a rotor hub. Before brazing the two parts together, both the end of the stem to be joined and the portion of the hub receiving the stem are given complimentary threads, which are used to screw the two parts together in a frictional joining arrangement. Voids are unavoidably created between these threads after the parts are screwed and then brazed together, temporarily trapping atmospheric gasses present during tube assembly. These gasses can later escape the voids during tube operation and cause undesirable outgassing that adversely affects the performance of the x-ray device.
When such brazing techniques are used to connect other x-ray tube components, similar problems are also encountered. For example, brazing is often used to bond the target layer material to an underlying substrate material to form the anode structure. Again, the resulting joint is especially susceptible to high thermal stresses, and thermal mismatches between the target material, the braze bond, and the underlying substrate often cause cracks to form within the anode, ultimately resulting in anode failure, or decreasing its operating efficiency. Moreover, the brazing or casting process typically introduces a change in microstructural textures of the adjacent materials, such as recrystallization and grain growth. Again, this phenomena can result in the formation of microcracks, which ultimately can lead to a failure of the device. Grain growth can also reduce the strength and ductility of the adjacent materials
Conventional brazing techniques used in an x-ray tube environment suffer from other drawbacks as well. In particular, the process involves a fairly complex and expensive manufacturing process. For example, there is typically a need for extensive chemical cleaning and/or purified water ultrasonic cleaning before a brazing process. Also, typically extensive high temperature outgassing runs are needed to keep the surfaces of the materials clean before brazing. Moreover, the process can only be done in a piece-by-piece manner, which limits the quantity of parts that can be manufactured, thereby increasing costs. Testing of a brazed joint is also difficult, and often there is no sure way to examine the integrity of the bond unless complex and costly testing procedures and equipment are used.
In light of the foregoing, a need therefore exists for a method of manufacturing x-ray tube components that provides an improved manner for bonding together x-ray tube components.
Briefly summarized, presently preferred embodiments of the present invention are directed to an improved x-ray tube in which selected components are secured together using an explosion bonding process. The preferred explosion bonding process utilizes the highly compressive force that accompanies a controlled explosion to push two materials together such that they form between them a high strength, metallurgical bond. The characteristics of the resulting bond are especially desirable for use in an x-ray tube environment because it is better able to withstand the environmental conditions of an x-ray tube.
While the disclosed bonding process can be used to attach a number of different tube components, in one preferred embodiment, the process is used to bond the stem and the rotor hub portions of an x-ray tube. The resulting bond is desirable in a number of respects. First, the resulting metallurgical bond interface between the stem and the rotor hub is characterized by the absence of a continuous intermetallic compound layer. Instead, only discrete particles of intermetallic materials are found in the joint region. The metallurgical bond, together with the absence of a continuous intermetallic layer, makes the explosively bonded joint much stronger than a conventional brazed joint.
In addition, the process provides a stronger bond. The joint between the explosively bonded stem and rotor hub is characterized by a peak-and-valley, or wave-like pattern at the interface. This characteristic is a byproduct of the explosion bonding process and effectively increases the surface area over which the two dissimilar materials may bond, thereby increasing the bond strength. It also results in a mechanically stronger joint under shear stress, because the wave pattern provides a screw thread-type interface. In contrast, a brazed joint interface is flat, and may more easily yield to shear stresses that can result from different thermal expansion rates of the materials, or the high rotational speeds of the anode.
Also, while in one preferred embodiment the explosion bonding process is used to connect the stem portion to the rotor hub portion, in other embodiments the process is used to connect different tube components. For instance, the process can be used to attach a rotor sleeve portion to the rotor hub portion. Or, the process can be used to bond a target surface material to an underlying substrate to form a rotating or stationary anode.
The explosion bonding process is also advantageous in a manufacturing sense. The process reduces the need for extensive cleaning of the parts before bonding, nor is there a need for extensive outgassing runs before bonding. Further, depending on the components being manufactured, the process is amenable to the production of a large quantity of parts such as a batch process, as opposed to the piece-by-piece production steps involved with brazing.
To summarize, use of an explosive bonding method for bonding x-ray tube components together addresses a number of problems. In particular, the approach provides excellent bonding characteristics for attaching various x-ray tube components and parts that are particularly susceptible to the harsh thermal and mechanical stresses in an operating x-ray tube. Parts such as the stem and rotor hub, anode target to anode substrate, and rotor sleeves to rotor jackets, and other component parts of the x-ray tube assembly, all benefit from the improved bond provided by explosion bondingxe2x80x94both in a manufacturing and a structural sense. Specifically, the resulting joint is sufficiently strong to prevent the formation of microcracks therein, and that is better able to withstand shear stresses. This extends the useful life of the tube components, insures increased operating efficiency, and contributes to an increased x-ray tube performance and lifetime.
These and other advantages and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.